In your equations, you have, for example,

(CASA[0] - CSatASA)^c

In the initial conditions, CASA[0] == 0, so this value turns out to have a negative base.

(CASA[0] - CSatASA)^c /. vals /. CASA[0] -> 0

Since the exponent 67/50 has an even denominator, there is no way to avoid a complex value, no matter which of the 50 roots is used. What value would you like it to have? Perhaps you meant to type

(CSatASA - CASA[t])^c

which would be similar to another factor that occurs in your code:

(CSatSA - CSA[t])^d